Philosophical State of the Physical Sciences. 495 



condensed air is increased, and the pitch of the sound is raised. 

 Possibly the colour of light would follow the same law; but there 

 is no experiment to prove it, and very little analogy exists be- 

 tween the eye and the ear. There is no analogy whatever be- 

 tween the subjective sensation by either organ and the physical 

 action of the prism. The questions at issue are these: — Does re- 

 fraction depend upon the absolute or the relative velocity of 

 light ? are the time of oscillation of the particles of aether and the 

 normal wave-length, corresponding to it, changed by any motion 

 of translation in the origin ? or is the conservation of these ele- 

 ments an essential attribute of the luminiferous medium ? It 

 has been said that Doppler reasoned as if the corpuscular theory 

 of light were true, and then expressed himself in the language 

 of undulations. Evidently there is an obscurity in the minds 

 of many physicists, and an uncertainty in all, when they reason 

 upon the mechanical constitution of the aether, and the funda- 

 mental laws of light. The mathematical theory is not so clear 

 as to be able to dispense with the illumination of experiment. 

 Within the present year, Van der Willigen has published a long 

 and well-considered memoir on the theoretical fallacies which 

 vitiate the whole of Huggins's argument for the motion of the 

 stars and nebula?. His analysis proves that the motion of the 

 luminary will not interfere with the time of oscillation and the 

 wave-length, provided that the origin of the disturbance is not a 

 mathematical point but a vibrating molecule, and that the sphere 

 of action of this molecule upon surrounding molecules is large 

 enough to keep them under its influence during ten or a hun- 

 dred vibrations, before it is withdrawn by the motion of transla- 

 tion. If this theoretical exposition of the subject should be 

 generally adopted by mathematicians, the spectroscopic obser- 

 vations on the supposed motion of the stars must receive another 

 interpretation. On the other hand, if a luminary is selected 

 which is known to move, independently of spectroscopic obser- 

 vations 5 and the displacement of the spectrum-lines accords with 

 this motion, it will be time to reconsider the mathematical theory, 

 and make our conceptions of the sether conform to the experiment. 





 The spectroscopic observation of Angstrom on an oblique electric 



spark does not favour Huggins's views. Secchi testifies to opposite 

 displacements when he examined, with a direct-vision spectro- 

 scope, the two edges of the sun's equator, one of which was 

 rotating toward him and the other from him ; and Vogel has re- 

 peated the observation with a reversion-spectroscope. This would 

 have the force of a crucial experiment were it not that an equal 

 displacement was seen on other parallels of latitude, and that 

 the bright bands of the chromosphere were moved, but not the 

 dark lines of the solar atmosphere. 



